I would love some suggestions for organ/violin duet music (or pieces adapted for organ/violin), and/or organ solo piece suggestions, especially things that would seem fitting for either communion or postludes, during the Christmas and Easter seasons. What are your favorites?

First, an answer to the question you posed: Le Cygne ... but the composer escapes me. I had to endure it repeatedly for 4 years. (I say endure not because it's an evil piece, but because it wears thin after a while.)

Second, assuming this is for the OF, I think you'll run into two roadblocks, from very different quarters.

a) from the "the people have to be doing something all the time" group, an organ/violin duet detracts from the people's participation.

b) from the "Sing the Propers, already" coetus, organ/violin doesn't work because the violin is usually unsuited to the liturgy. (There are exceptions, but a general rule would apply here).

I should clarify: this is for an FSSP parish. I am especially looking for some ideas for Christmas midnight mass with organ/violin (I would like a piece possibly as an interlude during our hour of carols before midnight mass begins; the violinist will be accompanying alot of Christmas carols w/ the organist as well, that's his main function), and then just organ for the morning mass, and we will have the organist for the whole Christmas season and feast days therein.

I usually have just let our organist play what he wants, knowing that our priests prefer more "conservative" selections (so usually no organ music characteristic of post-1900), but as our music program grows, would like to give him more direction regarding the instrumental music for the festive seasons.

He subs for us regularly, but sometimes plays the same things on different occasions; we don't have an in-house organist, and instrumental music is not my area of knowledge. So, with that said, looking for suggestions!

I'll be following this post. We have an in-house violinist and a cellist (though carrying and storing the cello in a small loft is sometimes difficult) and it would be nice to have instrumental music as you mentioned above.

canadash, I can say that I've found there are some duets by Rheinberger for violin/organ. But they aren't quite what I'm looking for, especially for postludes. I want something more festive for that. Here's the selections by Rheinberger:Six Pieces for Violin and Organ Op. 150Suite for Violin and Organ Op. 166.

Might I suggest you consider pieces by Biber. He wrote some very great music for violin with organ accompaniment that are somewhat religious. They can be though somewhat challenging and a cello can play the bass line of the accompaniment if desired. You can hear them on youtube. The music can be found on the IMSLP Free Music site, just look up the composer Biber.

Many sonatas for violin (or flute, or so forth) and continuo by Bach, Handel, and other composers of that period can work very well with organ. Sometimes, in editions realised for piano, one has to further edit so make the accompaniment more organistic and less pianoey, but this is not a great obstacle. These work very well as preludes, as wedding music - even, carefully chosen, for funerals.

We've often used Ann Callaway's Silent Twilight as an introduction to the post-communion Silent Night (it ends in B-flat). I can message a private email if you like. There's also a spookier take on Stille Nacht by A. Schnittge for violin & piano.

The local San Francisco AGO newsletter printed a manualiter arr. of the first allegro from Op. 6 No.8 some years ago; the Depolo keyboard score at IMSLP seems to be something else.

The Mozart church sonatas are great if you have two treble instruments. Baroque solo sonatas are usually the way to go.Canzonas by Frescobaldi are very liturgically suitable, however, it is hard to find continuo realizations for them.